Low-flow limit in slot coating: Theory and experiments

Citation
Ms. Carvalho et Hs. Kheshgi, Low-flow limit in slot coating: Theory and experiments, AICHE J, 46(10), 2000, pp. 1907-1917
Citations number
12
Categorie Soggetti
Chemical Engineering
Journal title
AICHE JOURNAL
ISSN journal
00011541 → ACNP
Volume
46
Issue
10
Year of publication
2000
Pages
1907 - 1917
Database
ISI
SICI code
0001-1541(200010)46:10<1907:LLISCT>2.0.ZU;2-9
Abstract
The region of acceptable duality in the space of operating parameters of a coating process is called coating window. Their limits are set by coating d efects. For the slot-coating process the low-flow limit is important. It co rresponds to the maximum web speed at a given film thickness, or the minimu m film thickness at a given web speed, at which the coating bead remains st able. The available viscocapillary model is based on the Landau-Levich equa tion, which is limited to small Capillary and Reynolds numbers. Under these conditions, the minimum film thickness that can be coated decreases with d ecreasing coating speed but many coating processes do not occur at low Capi llary numbers. It is important to determine the range of validity of the vi scocapillary model and find the low-flow limit outside this range. The low- flow limit was determined here theoretically and experimentally. The 2-D Na vier-Stokes equations with free surfaces describe liquid flow in the coatin g bead. Theoretical approaches solve the Navier-Stokes system by either usi ng Galerkin's method with finite-element basis functions or applying a long -wave expansion. The minimum layer thickness at a set of parameters was det ermined by the turning point on the solution path as the thickness was dimi nished The minimum film thickness was measured experimentally by determinin g the flow rate at which the coating bead breaks, leading to stripes of coa ted and uncoated web. Results show that the low-flow limit of the coating b ead at large Capillary and Reynolds numbers fundamentally differs from that at their low numbers. At large Capillary and Reynolds numbers, the minimum film thickness that can be coated decreases with increasing coating speed. The coating window of the process is much larger than that in the literatu re, broadening the applicability of this coating method.